Python 2.7 quick reference

16. Type dict: Dictionaries

Python dictionaries are one of its more powerful built-in types.
They are generally used for look-up tables and many similar
applications.

A Python dictionary represents a set of zero or more ordered
pairs (ki, vi) such
that:

Each ki value is called a
key;

each key is unique and immutable; and

the associated valuevi can be of any type.

Another term for this structure is mapping, since it maps the set of keys
onto the set of values (in the algebraic sense).

To create a new dictionary, use this general form:

{ k0: v0, k1: v1, ... }

There can be any number of key-value pairs (including zero).
Each key-value has the form “ki:vi”, and
pairs are separated by commas. Here are some examples of
dictionaries:

{}
{'Bolton': 'Notlob', 'Ipswich': 'Esher'}
{(1,1):48, (8,20): 52}

For efficiency reasons, the order of the pairs in a dictionary
is arbitrary: it is essentially an unordered set of ordered
pairs. If you display a dictionary, the pairs may be shown in a
different order than you used when you created it.